The present invention relates to a voltage/current converting circuit and a phase synchronizing circuit, and particularly relates to the phase synchronizing circuit using an electric current control oscillator, and the voltage/current converting circuit suitable for the phase synchronizing circuit.
The conventional phase synchronizing circuit (PLL circuit) uses a conventional electric current control oscillator.
In the conventional phase synchronizing circuit, an input signal (e.g., a reference clock signal) and an output signal of a frequency dividing circuit are inputted to a phase comparator. A phase difference is determined by the phase comparator. A control voltage is changed (smoothed) by charging or discharging an electric current according to a phase comparing result (phase difference information) to a loop filter by a charge pump circuit. A voltage control oscillator generates an oscillating signal having a frequency according to the control voltage smoothed by the loop filter. The oscillating signal is divided into N-frequencies by the frequency divider, and is fed back to the phase comparator.
An output signal (oscillating signal) from the voltage control oscillator and an output signal (frequency dividing signal) from the frequency dividing circuit are output to the outside as an output signal synchronized with the input signal to the phase synchronizing circuit from the phase synchronizing circuit. An electric current control oscillator is provided such that the voltage control oscillator includes the electric current control oscillator and a voltage/current converting circuit (V-I converter).
Namely, the voltage/current converting circuit converts the control voltage smoothed by the loop filter to a control electric current. The electric current control oscillator generates an oscillating signal having a frequency according to the control electric current.
There is also a phase synchronizing circuit having no frequency dividing circuit. If the phase comparator and the loop filter have a specific construction, no charge pump circuit is required therein. Therefore, no charge pump circuit is described in many cases in a block diagram as a constructional element within the phase comparator and the loop filter even when the charge pump circuit is required.
In the voltage control oscillator, a frequency control electric current from the voltage/current converting circuit is different in accordance with changes in a temperature condition, a process condition, a power supply voltage, etc. even when the oscillating signal of the same oscillating frequency is outputted.
Further, in the electric current control oscillator, it is desirable to set a frequency variable range to shorten a lock time, and avoid the oscillation at a frequency greatly shifted from an expected frequency. In this case, a circuit construction for limiting the electric current is used in the voltage/current converting circuit.
However, the number of satisfiable voltage/current converting circuits with the electric current limit was conventionally small.
Namely, in many conventional voltage/current converting circuits with the electric current limited, no linearity of input voltage and output electric current characteristics can be held near the limit electric current. A voltage frequency conversion coefficient of the voltage control oscillator is greatly differently seen in case the voltage control oscillator utilizes the voltage/current converting circuit. Further, even when the loop filter of the same constant is used, a problem exists in that no phase synchronizing circuit can be set to a lock state when the frequency control electric current reaches the vicinity of the limit electric current by the power voltage change, the temperature change and the process variation.
Therefore, the voltage/current converting circuit having a wide range of the linearity of the input voltage and output electric current characteristics is desired. Further, the phase synchronizing circuit able to preferably perform a phase synchronizing operation is desired even when the frequency control electric current from the built-in voltage/current converting circuit is close to the limit electric current.